EP1257079B1 - Système de transmission optique utilisant un traitement optique du signal dans des terminaux pour améliorer les performances du système - Google Patents

Système de transmission optique utilisant un traitement optique du signal dans des terminaux pour améliorer les performances du système Download PDF

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EP1257079B1
EP1257079B1 EP02253167A EP02253167A EP1257079B1 EP 1257079 B1 EP1257079 B1 EP 1257079B1 EP 02253167 A EP02253167 A EP 02253167A EP 02253167 A EP02253167 A EP 02253167A EP 1257079 B1 EP1257079 B1 EP 1257079B1
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Prior art keywords
signal
optical
optical signal
polarization
modulator
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German (de)
English (en)
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EP1257079A3 (fr
EP1257079A2 (fr
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Neal S. Bergano
M. Imran Hayee
Alexei N. Pilipetskii
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SubCom LLC
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Tyco Electronics Subsea Communications LLC
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/66—Non-coherent receivers, e.g. using direct detection
    • H04B10/67—Optical arrangements in the receiver
    • H04B10/671—Optical arrangements in the receiver for controlling the input optical signal
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • H04J14/03—WDM arrangements
    • H04J14/0305—WDM arrangements in end terminals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/06—Polarisation multiplex systems

Definitions

  • the present invention relates to an optical fiber communication system that carries multiple optical signals in corresponding channels using wavelength division multiplexing technology.
  • an improved optical receiver permits the use of lower signal-to-noise ratios in each WDM channel so that more margin is available to improve system performance.
  • a single fiber is used to carry multiple optical channels known as wavelength division multiplexing (hereinafter a WDM system).
  • WDM system wavelength division multiplexing
  • the fiber In a fiber optic network, the fiber itself has associated nonlinearities. At high optical signal powers, the fiber induces phase shifts on the optical signal due to these fiber nonlinearities. The induced phase shifts in the optical signal correspond to wavelength modulation imposed on the optical signal. When different portions of an optical signal have different wavelengths, these different portions propagate along the transmission fiber at different velocities due to dispersion properties inherent in the fiber media. After propagation for a distance, faster portions may overtake and become superimposed on slower portions causing amplitude distortion.
  • an optical phase modulation is sometimes imposed on the optical signal at the transmitter in what is referred to as chirped RZ (CRZ).
  • chirped RZ The inherent band spread of the chirped RZ waveform imposes a limit on how closely adjacent WDM channels may be spaced and subsequently the number of channels within a particular spectral band.
  • receivers including compensation for dispersion effects using polarisation control are shown in Morita et al, "40 GBIT/S single channel soliton transmission using periodic dispersion compensation” IEICE transactions on Electronics, Institute of Electronics Information and Comm. Eng. Tokyo, JP, vol. E81-C, no. 8, August 1998 (1998-08), pages 1309-1315, XP000848536 ISSN: 0916-8524 and (NEC Corp. ,) EP 0562514A1 .
  • Q-Factor is a measurement of the electrical signal-to-noise ratio at a receive circuit in a communication system that describes the system's bit error rate (BER) performance.
  • Q is inversely related to the BER that occurs when a bitstream propagates through the transmission path.
  • the BER increases at low signal-to-noise ratios (SNRs) and decreases at high SNRs.
  • SNRs signal-to-noise ratios
  • a BER below a specified rate can be achieved by designing the transmission system to provide an SNR greater than a predetermined ratio.
  • the predetermined SNR is based on the maximum specified BER. To achieve a low BER, the SNR must be high, and this may require that the signal power be at a level that induces undesired phase distortions due to fiber nonlinearities.
  • FEC Forward Error Correction
  • a system and method for transmitting and propagating closely spaced optical channels of a WDM system at lower signal-to-noise ratios than is used in known systems while maintaining the same or lower bit error rates.
  • An optical receiver in accordance with the present invention includes an optical pre-processor, an optical polarization section having a polarization rotator and an optical polarizer; and characterised by: a phase modulation section coupled to an output of said optical polarization section, said phase modulation section configured to compress said output of said polarization section and comprising a phase modulator and a dispersive fiber coupled to an output of said phase modulator; and an amplitude modulation section coupled to an output of said optical polarization section, said amplitude modulation section including an amplitude modulator.
  • optical communication systems can be improved by operating the system at lower SNRs in combination with FEC technology.
  • signals propagating through a WDM communication system with lower transmission path average power experience less nonlinear impairments. Therefore, channel spacings among WDM signals may be reduced thereby increasing system capacity.
  • optical network 100 includes terminal end 130, terminal end 120 and a plurality of repeaters (optical amplifiers) 110 linked by optical cable 106 and 107.
  • Terminal 130 and terminal 120 may each include receivers and transmitters to accommodate bidirectional transmission.
  • the optical signals that propagate over network 100 could do so at lower SNRs while maintaining adequate BER.
  • the output power of each of the optical amplifiers 110 is reduced such that a signal having a lower SNR is received at a receiving end (e.g. terminal 120) of the network.
  • the amount of phase modulation performed at the transmitter may be reduced, thus allowing for greater spectral efficiency and consequently greater channel capacity over a communications network.
  • FIG. 2 illustrates an exemplary terminal 130 of network 100 that includes transmitter 140 and combiner or multiplexer 132. It should be understood that the exemplary transmitter may also be included in terminal 120 for bidirectional transmission.
  • Combiner 132 combines one or more optical channels carried on respective one or more fibers 134 from respective one or more optical transmitters 140. These WDM signals propagate over network 100 via optical cable 107.
  • FIG. 3 illustrates an exemplary receiver circuit included in terminal 120 of network 100 which comprises a demultiplexer 122, optical preprocessor 170, photo detector circuit 127 and low pass filter 129. It should be understood that the exemplary receiver may also be included in terminal end 130 for bidirectional transmission.
  • Demultiplexer 122 divides terminal input signals received over cable 108 into individual wavelengths or channels. Once separated, each channel is processed through optical pre-processor circuit 170 to produce higher fidelity signal 125.
  • Photo detector circuit receives optical signal 125 and generates an electrical signal that corresponds to the optical signal carried in a particular one of the WDM channels.
  • FIG. 4 illustrates transmitter 140 which includes light source 142, data modulator 144, amplitude modulator 146 and phase modulator 148.
  • Laser or light source 142 provides a coherent light signal 150 to on-off data modulator 144 which provides optical on-off data signal 152 to amplitude modulator 146.
  • Amplitude modulator 146 provides AM modulated (e.g., shaped) optical signal 154 to phase modulator 148.
  • Phase modulator 148 provides an output optical signal on optical cable 134.
  • a band pass filter (not shown) may be provided prior to wavelength multiplexer 132 to reduce inter-channel interference.
  • Laser source 142 provides optical signal 150 at the nominal wavelength of transmitter 140 (or some constant offset therefrom depending on the specific implementations of modulators 144, 146 and 148). For example, laser source 142 may provide optical signal 150 at a wavelength of 1541 nanometers.
  • Amplitude modulator 146 shapes the power envelope of optical signal 152 so as to provide shaped optical signal 154.
  • Amplitude modulator 146 may include such shaping circuits as required to transform the clock signal input into a signal which drives the actual amplitude modulator to achieve the desired shaped optical signal.
  • Phase modulator 148 responds to a clock signal input to generate a "chirped" signal 134.
  • Modulator 148 imparts an optical phase angle that is time varying thereby imparting a frequency shift (and corresponding wavelength shift).
  • phase modulator 148 may vary the instantaneous wavelength between 1540.7 and 1541.3 nanometers (i.e., a 0.6 nanometer band spread) when laser 142 is providing an optical signal at 1541 nanometers.
  • on-off modulated optical signal 152 provided by data modulator 144 is depicted as having either on power or off power.
  • the electrical data signal provided into data modulator 144 is a bitstream of data encoded into a non-return to zero electrical signal (NRZ signal).
  • a "one” may be represented by 5 volts, and a "zero” is represented by 0 volts.
  • On-off modulator 144 modulates optical signal 150 to be optically "off” (e.g., no optical signal is output,) when the data bitstream into data modulator 144 is a "zero” and optically “on” (e.g., laser signal) when the data bitstream is a "one.”
  • Optical signal 152 depicts a non-return to zero (NRZ) coded optical signal (e.g., with a 100% pulse duty factor), where the four bits represent "1101."
  • Optical signal 154 illustrated in Fig. 5B , depicts a return to zero (RZ) coded optical signal after amplitude modulator 146.
  • Phase modulator 148 produces a chirped optical signal 134 with a corresponding instantaneous frequency deviation shown in FIG. 5C .
  • Repeaters 110 interspaced along cable 107 include amplifiers that amplify the transmitted optical signals incident thereon from terminals 120 and 130.
  • the amplifiers are erbium doped fiber amplifiers configured to provide amplification or gain at wavelengths within the low loss window of the optical fiber contained in cable 107 (e.g. 1550 nm region).
  • These amplifiers include erbium-doped fiber that is "pumped" with light at a selected wavelength, e.g., 980 nm, which excites the erbium and amplifies the incoming optical signal.
  • the spectral region for such amplifiers is typically between 1525 to 1573 nm.
  • these amplifiers also add unwanted noise (amplified spontaneous emission) to the signal.
  • a receiver in accordance with the present invention operates at lower SNR values while maintaining adequate bit error ratio performance.
  • the performance of an optical communication system can be improved by using a receiver in accordance with this present invention to receive optical signals having lower SNRs (resulting from lower launch power) which reduces phase distortions associated with fiber nonlinearities.
  • the SNR of the received signal is also reduced which would ordinarily lead to higher bit error rates (BERs).
  • data processing techniques such as forward error correction (FEC)
  • FEC forward error correction
  • the loss of bandwidth associated with the use of FEC is less than the gain in data bandwidth that results from providing more data channels.
  • FIG. 6 illustrates an optical pre-processor circuit 170 associated with a receive circuit shown in FIG. 3 in accordance with the present invention.
  • Circuit 170 includes band pass filter 176, optical polarization section 172, phase modulation section 174, and/or amplitude modulation section 178.
  • Band pass filter 176 is configured to pass one or more optical channels received via demultiplexer 122. It is understood by those skilled in the art that the channel selection function provided by 176 could also be part of demultiplexer 122.
  • Optical polarization section 172 includes polarization rotator 160, optical polarizer 162 and circuitry 165.
  • Polarization rotator 160 (sometimes called a polarization controller) rotates the polarization of the input optical signal according to an input control signal received from circuitry 165.
  • Optical tap 164 taps a portion (e.g. 5-10%) of the optical signal from polarizer 162 and supplies this signal to circuitry 165.
  • Circuitry 165 generates a polarization control signal to control polarization rotator 160.
  • the signal polarization is aligned with optical polarizer 162 to pass an associated portion of the optical signal to phase modulation section 174.
  • the portion of the optical signal orthogonal to polarizer 162 is not permitted to pass through polarizer 162. In this manner, the optical noise component associated with the orthogonal signal not passed through polarizer 162 is suppressed.
  • Transmitter 140 may be configured to take advantage of orthogonal polarization launch, where the WDM channels are launched into the system in a "pair-wise" orthogonal relationship as disclosed in U.S. Patent No.: 6,134,033 entitled “Method and Apparatus for improving Spectral Efficiency in Wavelength Division Multiplexed Transmission Systems.” If pair-wise orthogonal launch is employed then polarization section 172 of preprocessor circuit 170 passes the channel to be selected and filters unwanted adjacent channels. For example, if the receiver was configured to receive channel 10 having wavelength ⁇ 10 , then polarization section 172 would filter adjacent channels 9 having wavelength ⁇ 9 and channel 11 having wavelength ⁇ 11 .
  • Phase modulation section 174 includes dispersion element 180, phase modulator 182 and a clock recovery circuitry 185.
  • Dispersion element can be, for example, a length of dispersion compensating fiber.
  • Optical tap 184 senses an optical signal in the phase modulation section and circuitry 185 receives a portion of the optical signal from section 172 via optical tap 184 and provides a control signal that is adjustable in time relative to a clock signal derived from the tapped optical signal and/or adjustable to control a phase modulation index affected by phase modulator 182.
  • Circuitry 185 may include optical-to-electrical converter (e.g., a detector), adjustable delay circuit and driver.
  • circuitry 185 could be used to provide an electrical signal to phase modulator 182 that is functionally related to the optical intensity of the signal detected at tap 184; thus providing a phase modulation dependent on the intensity of the signal.
  • Phase modulation section 174 functions as a pulse compressor by exploiting the characteristic of dispersion element 180. Normal fiber media propagates optical signals at a rate that is dependent on the signal wavelength. The characteristic dispersion defines the rate of propagation as a function of the wavelength of the optical fiber. Dispersion element 180 has dispersion characteristics defined over particular wavelengths.
  • phase modulator 182 is controlled to impart, on the optical signal supplied by polarization section 172, a wavelength that varies from the start of the pulse to the end of the pulse as a result of modulation by phase modulator 182. Due to this variation in wavelength, there is a difference in the rate of optical signal propagation in dispersion element 180 experienced at the start of the pulse and at the end of the pulse such that the end of the pulse "catches up” with the pulse starting portion after passing through dispersion element 180.
  • a 50% CRZ signal i.e., an Full RZ signal (FRZ)
  • FRZ Full RZ signal
  • HRZ Half RZ signal
  • any reasonable compression ratio may be achieved in phase modulation section 174 when operated as a pulse compressor.
  • a compression factor of 1.0 means no compression at all.
  • a compression factor of 2.0 means compressing a 50% CRZ waveform into a 25% CRZ waveform.
  • a compression factor of 3.0 means compressing a 50% CRZ waveform into a 16-2/3% CRZ waveform.
  • the pulse width output from pulse compressor 174 is the pulse width into pulse compressor 174 divided by the compression factor.
  • the relative bit error rate performance of compressed pulses when compared to an uncompressed pulse is that the pulse compression leads to larger eye openings for better noise margin in the detection process. Narrower pulses in the channel detection process result in better channel Q due to the better eye opening. RZ pulses are better than NRZ pulses, and HRZ pulses are better than RZ pulses since HRZ pulses are narrower than RZ pulses. By compressing the input RZ pulses into narrower RZ pulses, signals with lower SNR may used without compromising the resulting bit error rate.
  • Amplitude modulation section 178 includes amplitude modulator 192, clock recovery circuitry 195 and spectral shaping filter 199.
  • Optical tap 194 taps a portion of the optical signal to circuitry 195 and spectral shaping filter 199.
  • Circuitry 195 is configured to provide a control signal, based on the portion of the optical signal received via tap 194, to modulator 192.
  • the control signal is adjustable in time relative to a clock signal derived from the tapped optical signal and/or adjustable to control an amplitude modulation index affected by the amplitude modulator.
  • Amplitude modulator 192 may advantageously be used to further attenuate the optical signal at times in the timeline that are outside of the pulse that has been narrowed in the pulse compressor (e.g., the combination of dispersive fiber 180 and phase modulator 182 work together as a pulse compressor).
  • Filter 199 may include a spectral weight to emphasize or de-emphasize different parts of the optical spectrum.
  • circuitry 195 could be used to provide an electrical signal to amplitude modulator 192 that is functionally related to the optical intensity of the signal detected at tap 194.
  • amplitude modulation section 178 advantageously narrows the pulse so that photo detector ( FIG.
  • An optical receiver that includes at least two of optical polarization section 172, phase modulation section 174 and amplitude modulation section 178 need not include duplicate optical taps. Alternatively, a single tap, e.g. 164, may be used to provide a portion of the optical signal to each of the polarization section 172, phase modulation section 174, and amplitude modulation section 178 as illustrated in Fig. 7 .
  • optical signal 125 supplied by optical preprocessor 170 to photodetector circuit 127 of Fig. 3 imposes a corresponding electrical field in the photodetector circuit.
  • the electrical field comprises noise, hereinafter ⁇ noise>.
  • the electrical field is comprised of a ⁇ noise> component plus a binary one, hereinafter ⁇ one>, where data modulator 144 is "on".
  • Photodetector circuit 127 can be, for example, a square law detector of the electrical field.
  • the output of the detector is the square root of the noise component ( ⁇ noise> 2 ) when the signal represents a "zero.”
  • the electric field at the detector comprises ⁇ one>+ ⁇ noise>
  • the electrical signal output of the detector is the square root of [ ⁇ one>+ ⁇ noise>] 2
  • Q M 1 - M 0 ⁇ 1 + ⁇ 0
  • ⁇ one> 2 of equation (1) does not contribute to the expected standard deviation 1 since ⁇ one> is the deterministic value of a "one" without noise.
  • the terms 2 ⁇ one> ⁇ noise> and ⁇ noise> 2 of equation (1) contribute to the expected standard deviation 1 since ⁇ noise> is a stochastic value that varies from measurement to measurement.
  • the term "2 ⁇ one> ⁇ noise>" in equation (1) is referred to as variance ⁇ S- SP 2 (signal spontaneous noise) and the term “ ⁇ noise> 2 " is referred to as variance ⁇ SP-SP 2 (spontaneous to spontaneous noise)
  • the variance of all measurements of a "one" attributable to noise is designated as ⁇ 1 2 and equals the sum of variance signal to spontaneous noise ( ⁇ S-SP 2 ) and variance spontaneous to spontaneous noise ⁇ ( SP-SP 2 ).
  • the variance of all measurements of a "zero" attributable to noise is designated as ⁇ 0 2 and equals variance spontaneous to spontaneous noise ( SP ⁇ ⁇ SP 2 ).
  • the voltage after the low pass filter 129 is measured at the center of the bit space for each received bit. After a large number of bits are measured in this way, statistical averages and variances may be computed.
  • the arithmetic mean of all "ones” i.e., the mean of the square root of [ ⁇ one>+ ⁇ noise>] 2
  • M 1 the mean of all "zeros” (i.e., the mean of ⁇ noise>) is defined to be M 0 .
  • preprocessor 170 reduces the effects of ⁇ SP-SP 2 noise by (i) blocking one polarization (orthogonal to the signal) using polarization section 172, and (ii) compressing the optical signal pulse in time domain by phase modulation using phase modulation section 174.
  • FIG. 8 illustrates simulation results of Q associated with a channel with a 100 GHz optical bandpass filter carrying 10 Gb/s of data at signal-to-noise ratios (SNRs) ranging from -3.4 dB to 4.6 dB.
  • SNRs signal-to-noise ratios
  • FIG. 9 illustrates the simulation results of the improvement in dB of Q for a 100 GHz optical band pass channel carrying 10 Gb/s of data with optical pre-processor circuit 170 relative to the Q for the same 100 GHz channel without optical pre-processor circuit 170. Also shown is the simulation results of the improvement (increase) of Q for a 50 GHz band pass carrying 10Gb/s of data channel with optical pre-processor circuit 170 as compared to Q associated with the same 50 GHz channel without optical pre-processor circuit 170. Both are graphed to show the results at signal-to-noise ratios (SNRs) ranging from -3.4 dB to 4.6 dB.
  • SNRs signal-to-noise ratios
  • the improvement in Q from the use of optical pre-processor circuit 170 varies from 2.2 dB to 0.85 dB.
  • the improvement in Q from the use of optical pre-processor circuit 170 varies from 1.5 dB to 0.6 dB.
  • optical pre-processor circuit 170 By using optical pre-processor circuit 170, Q associated with a WDM channel can be improved. Pulse compression from phase modulator section 174 minimizes spontaneous - spontaneous beat noise (i.e., ⁇ SP-SP 2 noise) and improves receiver sensitivity. At SNR ranges that produce Q values between 13 and 14 dB ( FIG. 8 ), more than 1dB of improvement in Q can be obtained by using optical pre-processor circuit 170.
  • optical pre-processor circuit 170 With the addition of optical pre-processor circuit 170 and the attending improvement in Q, the bit error rate is reduced. For example (refer to FIG. 9 ), in a 100 GHz channel operating at a signal-to-noise ratio (SNR) of about 3 dB, the addition of optical pre-processor circuit 170 increases the channel Q (and therefore, lowers the bit error rate) by about 1.0 dB.
  • SNR signal-to-noise ratio
  • the improvement in Q in a lower SNR operating environment indicates that the benefit of using pre-processor 170 is larger in a lower SNR environment as compared to a higher SNR operating environment.
  • FIG. 10 illustrates simulation results of Q for input signal-to-noise ratios of 3, 6, 9, 12 and 15 dB associated with three pulse compression ratios: NRZ (100% duty cycle), full RZ (50% pulse duty cycle) and half RZ (25% higher pulse duty cycle).
  • NRZ 100% duty cycle
  • full RZ 50% pulse duty cycle
  • half RZ 25% higher pulse duty cycle
  • the noise bandwidth is defined by low pass filter 129 ( FIG. 2 ) with a bandwidth, in this simulation, of 5 GHz for normal NRZ, 6 GHz for full RZ (with chirp) and 12 GHz for half RZ.
  • FIG. 11 illustrates the gain in Q achieved by changing from NRZ to full RZ and the gain in Q achieved by changing from full RZ to half RZ.
  • the Q associated with full RZ is about 1.7 dB greater than the Q associated with 1 NRZ at a high (i.e., 15 dB) SNR, and this gain in Q increases still further to about 2.4 dB when the SNR is decreased to a low (i.e., 3 dB) SNR.
  • Q associated with half RZ is about 0.4 dB greater than Q for full RZ at a high (i.e., 15 dB) SNR, and this gain in Q increases still further to about 1.8 dB when the SNR is decreased to a low (i.e., 3 dB) SNR.
  • the improvement in Q when using optical pre-processor circuit 170 in the receiver expands at lower SNRs when compared to higher SNRs.
  • the receiver in accordance with the present invention is configured to work with standard power levels, but will have a larger Q-factor enhancement in a lower SNR environment.
  • Forward error correction (FEC) processing technologies may be used to recover any adverse change in bit error rate, if an adverse change occurs, associated with a change from a high SNR to a low SNR.
  • FEC Forward error correction
  • narrow RZ pulses are transmitted with the high peak powers associated with narrow pulses, gain in Q may be nullified with higher fiber non-linearity effects. These non-linearity effects result in a wavelength modulation imposed across the pulse width. Therefore, wider pulses are transmitted and propagated in the fiber, and pulse compression is performed at the receiver using pre-processor circuit 170.
  • phase modulation section 174 used 2 kilometers of dispersion compensating fiber 180 and phase modulator 182 was designed to modulate the optical phase up to two radians.
  • FIG. 12 illustrates Q results plotted for compression factors ranging from 1.0 to 3.0 at SNRs ranging from 2 to 17 dB.
  • Q tends to approach a single value (about 28 dB for an SNR of 17 dB).
  • the higher compression factors achieve noticeably higher Q.
  • system 100 By rejecting one polarization in optical polarizer 162 ( FIG. 6 ) and by compressing the optical pulses in phase modulation section 174 ( FIG. 6 ), system 100 ( FIG. 1 ) is able to maintain the same or reduced bit error rate for the desired data bit transport rate because of the improved Q achieved at the lower SNRs.
  • the lower SNR is achieved by either transmitting or amplifying at lower peak powers. By lowering the peak power, effects of fiber nonlinearities are minimized.
  • the phase modulation that is imposed by phase modulator 148 ( FIG. 4 ) to compensate for these nonlinear effects may be lessened so that the phase modulated optical signal varies over a narrower band of wavelengths. This permits adjacent channels in a wavelength division multiplex (WDM) system such as network 100 to be spaced on closer wavelength intervals.
  • WDM wavelength division multiplex
  • Existing communications systems that include an existing optical receiver may be improved by replacing it with an improved optical receiver in accordance with the present invention.
  • the improved optical receiver includes a pulse compressor to compress pulses of the propagating optical signal into a compressed pulse.

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Claims (8)

  1. Préprocesseur [170] utilisé dans un récepteur optique [120] comprenant :
    - une section de polarisation optique [172] possédant un rotateur de polarisation [160] et un polariseur optique [162] ; et caractérisé par :
    - une section de modulation de phase [174] couplée à une sortie de ladite section de polarisation optique, ladite section de modulation de phase étant conçue pour compresser ladite sortie de ladite section de polarisation et comprenant un modulateur de phase [182] et une fibre à dispersion [180] couplée à une sortie dudit modulateur de phase ; et
    - une section de modulation d'amplitude [178] couplée à une sortie de ladite section de polarisation optique, ladite section de modulation d'amplitude comprenant un modulateur d'amplitude [192].
  2. Préprocesseur selon la revendication 1, dans lequel la section de modulation de phase comprend en outre un circuit de récupération d'horloge [185] couplé pour contrôler le modulateur de phase.
  3. Préprocesseur selon la revendication 1, dans lequel la section de modulation d'amplitude comprend en outre un circuit de récupération d'horloge [185] couplé pour contrôler le modulateur d'amplitude.
  4. Préprocesseur selon la revendication 1, dans lequel la section de polarisation optique comprend en outre un circuit d'asservissement à la polarisation [165] couplé pour contrôler le rotateur de polarisation.
  5. Préprocesseur selon la revendication 1, comportant en outre un circuit de récupération d'horloge comprenant :
    - un point de connexion [184] destiné à prélever un signal optique, soit dans la section de modulation de phase, soit dans la section de modulation d'amplitude ; et
    - un circuit [185] destiné à fournir un signal de contrôle du modulateur de phase et un signal de commande du modulateur d'amplitude, le signal de commande du modulateur de phase étant au moins soit temporisable par rapport à un signal d'horloge issu du signal optique prélevé au point de connexion, soit réglable pour contrôler un indice de modulation de phase affecté par le modulateur de phase ; le signal du modulateur d'amplitude étant pour sa part au moins soit temporisable par rapport au signal d'horloge, soit réglable pour contrôler un indice de modulation d'amplitude affecté par le modulateur d'amplitude.
  6. Procédé de réception d'un signal optique comprenant les étapes consistant à :
    - démultiplexer un signal d'entrée en au moins un signal optique ;
    - prétraiter le premier signal optique du au moins un signal optique pour former un premier signal prétraité, l'étape de prétraitement comprenant les étapes consistant à faire pivoter la polarisation du premier signal optique, à compresser une impulsion du premier signal optique, et à moduler l'amplitude de l'impulsion du premier signal optique ; et
    - détecter le premier signal prétraité pour former un signal électrique, procédé dans lequel le rapport signal/bruit de référence est défini comme étant le rapport signal/bruit d'un signal reçu qui produirait un taux prédéterminé d'erreurs sur les bits dans le signal électrique en l'absence d'étape de prétraitement rapport signal/bruit ; un rapport signal/bruit atténué est inférieur au rapport/bruit de référence ; et
    le taux d'erreurs sur les bits dans le signal électrique est inférieur au taux d'erreurs sur les bits prédéterminé lorsque la réception dudit au moins signal optique s'effectue au rapport signal/bruit atténué,
    dans lequel l'étape de compression de l'impulsion consiste à :
    - récupérer un signal d'horloge à partir du premier signal optique ;
    - contrôler un modulateur de phase au moyen du signal d'horloge récupéré :
    (a) soit en retardant le signal d'horloge d'un retard réglable,
    (b) soit en donnant au signal d'horloge une forme réglable,
    et en faisant passer le premier signal optique à travers une fibre à dispersion.
  7. Procédé selon la revendication 6, dans lequel l'étape de récupération du signal d'horloge consiste à :
    - coupler une partie du premier signal optique pour former un signal optique prélevé ; et
    - détecter le signal optique prélevé pour former le signal d'horloge.
  8. Procédé selon la revendication 6, dans lequel l'étape de récupération du signal d'horloge consiste à :
    - coupler une partie du signal optique pivoté pour former un signal optique prélevé ; et
    - détecter le signal optique prélevé pour former le signal d'horloge.
EP02253167A 2001-05-07 2002-05-07 Système de transmission optique utilisant un traitement optique du signal dans des terminaux pour améliorer les performances du système Expired - Lifetime EP1257079B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US850600 1992-03-13
US09/850,600 US7203429B2 (en) 2001-05-07 2001-05-07 Optical transmission system using optical signal processing in terminals for improved system performance

Publications (3)

Publication Number Publication Date
EP1257079A2 EP1257079A2 (fr) 2002-11-13
EP1257079A3 EP1257079A3 (fr) 2003-02-05
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US20070009265A1 (en) 2007-01-11
US7336908B2 (en) 2008-02-26
EP1257079A3 (fr) 2003-02-05
EP1257079A2 (fr) 2002-11-13
US7203429B2 (en) 2007-04-10
US20020176144A1 (en) 2002-11-28

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